The present invention relates to a run flat tire in which the low exothermic property and high hardness of a side reinforcing layer are compatible and durability is improved.
At present, a run flat tire having a side reinforcing layer with high hardness which is arranged at the inside of a side wall portion is practically used and even if a car becomes in a state in which air pressure is lost by blowout, it has come to be able to run at a distance to a certain degree. Thereby, necessity to always provide a spare tire is extinguished and the weight saving of weight in the whole vehicle can be expected.
However, since the run flat tires support the weight of a car with a side reinforcing layer running at low inner pressure, there has been a problem that they are thermally deteriorated by repeating great deformation during running. Accordingly, speed and running distance were limited for the running of run flat tires while running at low inner pressure.
Although there is a method of suppressing deformation by thickening the side reinforcing layer and preventing fracture by deformation, as an effective method of improving the durability of run flat tires, there have been problems in that the weight of tires is enlarged which is contrary to the weight saving being the original purpose of run flat tires.
Further, there is also known a method of hardening a rubber for reinforcement by increasing a filler for reinforcement such as carbon black and suppressing the deformation of a side reinforcing layer, load for steps such as kneading and extrusion is great and heat generation is large in physical property after vulcanization, the improvement of the durability of run flat tires is not expected so much.
In the Japanese Unexamined Patent Publication No. 2002-155169, there is disclosed a low exothermic method of heightening the cure density of a side reinforcing layer by increasing the compounding amount of a vulcanizing agent and a vulcanization accelerator to the side reinforcing layer without increasing the amount of a filler for reinforcement, such as carbon black, and lessening the deformation quantity at the running of run flat tires at low inner pressure, but there have been problems that the elongation of the side reinforcing layer is lowered and strength at break is lowered.